The Synergistic Effect of Polystyrene/Modified Boron Nitride Composites for Enhanced Mechanical, Thermal and Conductive Properties.

Thermal conductivity (TC) and thermal stability are the basic requirements and highly desirable properties in thermal management, heat storage and heat transfer applications. This work is regarding the fabrication of polystyrene/boron nitride composites and melt extruded to produce good thermal stability, increased thermal conductivity and enhanced mechanical properties. Our strategy is potentially applicable to produce thermally conductive composites of low cost over large scale. Boron nitride powder is bath sonicated in 10% NH3 solution to avoid its agglomeration and tendency toward entanglement in a polymer matrix. An approximately 67.43% increase in thermal conductivity and 69.37% increase in tensile strength as well as 56 multiple increases in thermal stability of the optimum samples were achieved. The developed polymeric composites are potentially applicable in the electronic industry, especially in electronic devices used for 5G, heat sink and several other aviation applications.

Polystyrene-Sepiolite Clay Nanocomposites with Enhanced Mechanical and Thermal Properties.

Polystyrene (PS)/sepiolite clay nanocomposites were prepared via the melt extrusion technique using vinyl tri-ethoxy silane (VTES) as the compatibilizer and cross-linking agent. Mechanical, thermal, and flame-retardant properties of the newly developed polystyrene-based nanocomposites were determined. Surface morphology was investigated using scanning electron microscopy (SEM), examining the distribution of the filler in various compositions of fabricated composites. Structural analysis of the samples was carried out using the Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) techniques. Thermal stability was determined by thermal gravimetric analysis (TGA), showing a maximum 30.2 wt.% increase in residue by adding sepiolite clay. The results obtained from the dynamic mechanical analyzer (DMA) in terms of the storage modulus, loss modulus and damping factor exhibited better stress transfer rate and effective interfacial adhesion between the filler and the matrix. The higher filler loaded sample showed greater flame retardancy by decreasing the burning rate up to 48%.

Synthesis of Magnetic Fe3O4 Nano Hollow Spheres for Industrial TNT Wastewater Treatment.

The aim of the present work was to synthesize magnetite (Fe3O4) nano hollow spheres (NHS) via simple, one-pot, template-free, hydrothermal method. The structural, morphological, and surface analysis of Fe3O4 NHS were studied by scanning electron microscopy (SEM), x-ray diffraction technique (XRD), Fourier transform infrared spectroscopy FTIR and burner-Emmett-teller (BET). The as obtained magnetic (Fe3O4) NHS were used as an adsorbent for treating industrial trinitrotoluene (TNT) wastewater to reduce its Chemical Oxygen Demand (COD) values. Adsorption capacity (Qe) of the NHS obtained is 70 mg/g, confirming the attractive forces present between adsorbent (Fe3O4 NHS) and adsorbate (TNT wastewater). COD value of TNT wastewater was reduced to >92% in 2 h at room temperature. The adsorption capacity of Fe3O4 NHS was observed as a function of time, initial concentration, pH, and temperature. The applied Fe3O4 NHS was recovered for reuse by simply manipulating its magnetic properties with slight shift in pH of the solution. A modest decrease in Qe (5.0−15.1%) was observed after each cycle. The novel Fe3O4 NHS could be an excellent candidate for treating wastewater generated by the intermediate processes during cyclonite, cyclotetramethylene-tetranitramine (HMX), nitroglycerin (NG) production and other various environmental pollutants/species.

Mechanical characterization of brain tissue in tension at dynamic strain rates.

Mechanical characterization of brain tissue at high loading velocities is crucial for modeling Traumatic Brain Injury (TBI). During severe impact conditions, brain tissue experiences compression, tension and shear. Limited experimental data is available for brain tissue in extension at dynamic strain rates. In this research, a High Rate Tension Device (HRTD) was developed to obtain dynamic properties of brain tissue in extension at strain rates of ≤90/s. In vitro tensile tests were performed to obtain properties of brain tissue at strain rates of 30, 60 and 90/s up to 30% strain. The brain tissue showed a stiffer response with increasing strain rates, showing that hyperelastic models are not adequate. Specifically, the tensile engineering stress at 30% strain was 3.1±0.49kPa, 4.3±0.86kPa, 6.5±0.76kPa (mean±SD) at strain rates of 30, 60 and 90/s, respectively. Force relaxation tests in tension were also conducted at different strain magnitudes (10-60% strain) with the average rise time of 24ms, which were used to derive time dependent parameters. One-term Ogden, Fung and Gent models were used to obtain material parameters from the experimental data. Numerical simulations were performed using a one-term Ogden model to analyze hyperelastic behavior of brain tissue up to 30% strain. The material parameters obtained in this study will help to develop biofidelic human brain finite element models, which can subsequently be used to predict brain injuries under impact conditions and as a reconstruction and simulation tool for forensic investigations.

Mechanical characterization of brain tissue in simple shear at dynamic strain rates.

During severe impact conditions, brain tissue experiences a rapid and complex deformation, which can be seen as a mixture of compression, tension and shear. Diffuse axonal injury (DAI) occurs in animals and humans when both the strains and strain rates exceed 10% and 10/s, respectively. Knowing the mechanical properties of brain tissue in shear at these strains and strain rates is thus of particular importance, as they can be used in finite element simulations to predict the occurrence of brain injuries under different impact conditions. However, very few studies in the literature provide this information. In this research, an experimental setup was developed to perform simple shear tests on porcine brain tissue at strain rates ≤120/s. The maximum measured shear stress at strain rates of 30, 60, 90 and 120/s was 1.15±0.25kPa, 1.34±0.19kPa, 2.19±0.225kPa and 2.52±0.27kPa, (mean±SD), respectively at the maximum amount of shear, K=1. Good agreement of experimental, theoretical (Ogden and Mooney-Rivlin models) and numerical shear stresses was achieved (p=0.7866-0.9935). Specimen thickness effects (2.0-10.0mm thick specimens) were also analyzed numerically and we found that there is no significant difference (p=0.9954) in the shear stress magnitudes, indicating a homogeneous deformation of the specimens during simple shear tests. Stress relaxation tests in simple shear were also conducted at different strain magnitudes (10-60% strain) with the average rise time of 14ms. This allowed us to estimate elastic and viscoelastic parameters (initial shear modulus, µ=4942.0Pa, and Prony parameters: g1=0.520, g2=0.3057, τ1=0.0264s, and τ2=0.011s) that can be used in FE software to analyze the non-linear viscoelastic behavior of brain tissue. This study provides new insight into the behavior in finite shear of brain tissue under dynamic impact conditions, which will assist in developing effective brain injury criteria and adopting efficient countermeasures against traumatic brain injury.

Influence of preservation temperature on the measured mechanical properties of brain tissue.

The large variability in experimentally measured mechanical properties of brain tissue is due to many factors including heterogeneity, anisotropy, age dependence and post-mortem time. Moreover, differences in test protocols also influence these measured properties. This paper shows that the temperature at which porcine brain tissue is stored or preserved prior to testing has a significant effect on the mechanical properties of brain tissue, even when tests are conducted at the same temperatures. Three groups of brain tissue were stored separately for at least 1h at three different preservation temperatures, i.e., ice cold, room temperature (22 °C) and body temperature (37 °C), prior to them all being tested at room temperature (~22 °C). Significant differences in the corresponding initial elastic shear modulus µ (Pa) (at various amounts of shear, 0≤K≤1.0) were observed. The initial elastic moduli were 1043±271 Pa, 714±210 Pa and 497±156 Pa (mean±SD) at preservation temperatures of ice cold, 22 °C and 37 °C, respectively. Based on this investigation, it is strongly recommended that brain tissue samples must be preserved at an ice-cold temperature prior to testing in order to minimize the difference between the measured in vitro test results and the in vivo properties. A by-product of the study is that simple shear tests allow for large, almost perfectly homogeneous deformation of brain matter.

Temperature effects on brain tissue in compression.

Extensive research has been carried out for at least 50 years to understand the mechanical properties of brain tissue in order to understand the mechanisms of traumatic brain injury (TBI). The observed large variability in experimental results may be due to the inhomogeneous nature of brain tissue and to the broad range of test conditions. However, test temperature is also considered as one of the factors influencing the properties of brain tissue. In this research, the mechanical properties of porcine brain have been investigated at 22°C (room temperature), and at 37°C (body temperature) while maintaining a constant preservation temperature of approximately 4-5°C. Unconfined compression tests were performed at dynamic strain rates of 30 and 50s(-1) using a custom made test apparatus. There was no significant difference (p=0.8559-0.9290) between the average engineering stresses of the brain tissue at the two different temperature conditions. The results of this study should help to understand the behavior of brain tissue at different temperature conditions, particularly in unconfined compression tests.

Determination of friction coefficient in unconfined compression of brain tissue.

Unconfined compression tests are more convenient to perform on cylindrical samples of brain tissue than tensile tests in order to estimate mechanical properties of the brain tissue because they allow homogeneous deformations. The reliability of these tests depends significantly on the amount of friction generated at the specimen/platen interface. Thus, there is a crucial need to find an approximate value of the friction coefficient in order to predict a possible overestimation of stresses during unconfined compression tests. In this study, a combined experimental-computational approach was adopted to estimate the dynamic friction coefficient µ of porcine brain matter against metal platens in compressive tests. Cylindrical samples of porcine brain tissue were tested up to 30% strain at variable strain rates, both under bonded and lubricated conditions in the same controlled environment. It was established that µ was equal to 0.09±0.03, 0.18±0.04, 0.18±0.04 and 0.20±0.02 at strain rates of 1, 30, 60 and 90/s, respectively. Additional tests were also performed to analyze brain tissue under lubricated and bonded conditions, with and without initial contact of the top platen with the brain tissue, with different specimen aspect ratios and with different lubricants (Phosphate Buffer Saline (PBS), Polytetrafluoroethylene (PTFE) and Silicone). The test conditions (lubricant used, biological tissue, loading velocity) adopted in this study were similar to the studies conducted by other research groups. This study will help to understand the amount of friction generated during unconfined compression of brain tissue for strain rates of up to 90/s.

Mechanical characterization of brain tissue in compression at dynamic strain rates.

Traumatic brain injury (TBI) occurs when local mechanical load exceeds certain tolerance levels for brain tissue. Extensive research has been done previously for brain matter experiencing compression at quasistatic loading; however, limited data is available to model TBI under dynamic impact conditions. In this research, an experimental setup was developed to perform unconfined compression tests and stress relaxation tests at strain rates ≤90/s. The brain tissue showed a stiffer response with increasing strain rates, showing that hyperelastic models are not adequate. Specifically, the compressive nominal stress at 30% strain was 8.83 ± 1.94, 12.8 ± 3.10 and 16.0 ± 1.41 kPa (mean ± SD) at strain rates of 30, 60 and 90/s, respectively. Relaxation tests were also conducted at 10%-50% strain with the average rise time of 10 ms, which can be used to derive time dependent parameters. Numerical simulations were performed using one-term Ogden model with initial shear modulus µ(o)=6.06±1.44, 9.44 ± 2.427 and 12.64 ± 1.227 kPa (mean ± SD) at strain rates of 30, 60 and 90/s, respectively. A separate set of bonded and lubricated tests were also performed under the same test conditions to estimate the friction coefficient µ, by adopting combined experimental-computational approach. The values of µ were 0.1 ± 0.03 and 0.15 ± 0.07 (mean ± SD) at 30 and 90/s strain rates, respectively, indicating that pure slip conditions cannot be achieved in unconfined compression tests even under fully lubricated test conditions. The material parameters obtained in this study will help to develop biofidelic human brain finite element models, which can subsequently be used to predict brain injuries under impact conditions.